The present invention relates generally to the field of radio frequency (RF) ablation of tissue, and, more specifically, to devices and methods for the neddleless injection of a conductive fluid into tissue prior to and in conjunction with RF ablation.
RF ablation techniques destroy tissue by heating. Typically, high frequency alternating current flows from one or more electrodes into the tissue, producing ionic agitation in the tissue about the electrode area as the ions attempt to follow the direction changes of the alternating current. The agitation eventually drives water from the cells leading to desiccation and coagulation, thereby creating a lesion in the tissue. There are generally two forms of heating that lead to lesion formation in RF ablated tissue. The first is primary heating (direct) of tissue via the interaction of the tissue with the alternating current of the RF electrode. Tissue is also heated by secondary heating (indirect) via conductive heating from the areas heated by the RF energy. The size of the lesion depends on several factors including the amount of RF power delivered to the tissue, the amount of time the RF electrode is energized, as well as resistivity of the tissue (which can change as the tissue is coagulated).
RF ablation techniques are currently used to form lesions in a variety of organ and tissue types. For example, RF ablation has been used to form lesions in cancerous lung and liver tissue. One other important area where RF ablation is used relates to the treatment of cardiac arrhythmia. The heart includes a number of normal electrical pathways which are responsible for the propagation of electrical signals from the upper to lower chambers necessary for performing normal systole and diastole function. The presence of arrhythmogenic site or accessory pathway can bypass or short circuit these normal pathways, potentially resulting in very rapid heart contractions, typically referred to as tachycardias. Treatment is accomplished by ablating the abnormal arrythmogenic tissue or accessory pathway responsible for the tachycardia. RF ablation is a standard treatment for supraventricular tachycardia (SVT) such as antrioventricular nodal reentry tachycardia (AVNRT) and accessory pathways. RF ablation is also used in the treatment of ventricular tachycardia (VT) and atrial fibrillation (AF).
Current RF technologies deposit most of their delivered RF energy to within about 2 mm of tissue adjacent to where the ablation electrode is placed. Lesion depth is extended by thermal conduction (secondary heating) to deeper tissue layers over time. The maximum safe RF power that can be delivered is limited by the need to keep the maximum tissue temperature below 100xc2x0 C. If tissue temperature exceeds 100xc2x0 C., an explosive release of steam can erupt that could cause harmful and unwanted perforations in the tissue. Consequently, the maximum lesion depth that can be created using RF ablation devices that are not actively cooled is around 4-5 mm.
When an actively cooled RF ablation device is used, RF power can be increased as compared to non-cooled RF devices without exceeding the maximum tissue temperature of 100xc2x0 C. Ablation using cooled RF electrodes can result in lesion depths of 8-10 mm. However, a disadvantage of cooling the RF electrodes is that this exterior cooling does not greatly effect tissue temperature more than about 2 mm below the surface. While temperatures may be within acceptable limits at the surface, temperatures below the surface may exceed safety margins. Since the hottest tissue temperatures are located 1-3 mm below the surface, control of RF power to safe levels using active cooling is difficult.
Another technique that has been used to create deeper lesions is the irrigation and pumping of a saline solution directly into the tissue to be ablated. The irrigation is typically accomplished using hollow electrode needle-type structures that have holes drilled therein that allow saline solution to exit into the tissue of interest. These same needle-type structures are also used to deliver the RF energy during ablation. The injection of conductive fluid decreases electrical resistance (i.e., reduces ohmic losses) and thus permits the tissue to carry more energy without exceeding the 100xc2x0 C. upper temperature limit. The difficulty with this method lies in the unpredictability of the fluid transfer. Moreover, prior art devices typically delivery saline solutions at relatively low pressures, relying on the migration of the saline fluid through the extracellular space. Consequently, it is sometimes difficult to produce deep penetration of saline solution over a specific portion of the tissue of interest.
For example, experimental results using injection by needle of dyed saline solution indicate that injectate tends to flow in between tissue layers and could orient current in unexpected directions from the injection site. The conductive fluid, in other words, does not reliably go in a consistent pattern thus making a predictable and precise ablation of tissue ablation very difficult. Because of the unpredictable nature of injection of a conductive fluid via a needle, this is not an optimum approach in the treatment of a disease such as cardiac arrhythmia (i.e., in treatments where precision is important).
It is desirable, therefore, to improve RF ablation techniques so that deeper lesions can be created of a predictable size while at the same time keeping tissue temperatures below 100xc2x0 C. throughout the lesion area. As will be described in more detail below, the present invention provides improved lesion creation such that it achieves these and other desired results, which will be apparent from the description below to those skilled in the art.
In a first aspect of the invention, a method for preparing heart tissue for RF ablation includes the steps of providing an injection device adjacent to a portion of the heart tissue and injecting the tissue with a jet of conductive fluid to a depth within the range of about 2 mm to about 5 mm.
In a second aspect of the invention, a method for ablating heart tissue comprises the steps of providing an injector device adjacent to a portion of the heart tissue the injector device including a plurality of ports and a plurality of electrodes, injecting the heart tissue with a jet of conductive fluid from the injector device, and applying RF energy to the heart tissue containing the injected conductive fluid using the electrode so as to form a lesion.
In yet another aspect of the invention, a method of forming a lesion within heart tissue includes the steps of providing access to the heart of a patient, injecting a portion of the heart tissue with a jet of conductive fluid to a depth within the range of about 2 mm to about 5 mm, and ablating at least some of the tissue injected with the jet of conductive solution using RF ablation.
In still another aspect of the invention, a device is disclosed for injecting conductive solution into heart tissue. The device includes a source of pressurized gas, a syringe having at least one nozzle at one end thereof, the source of pressurized gas being in fluid communication with a plunger contained within the syringe. A conductive fluid is stored within the syringe. A switch is provided for releasing a pressurized gas from the source into the syringe so as to cause the device to deliver a jet of conductive fluid at a speed sufficient to penetrate heart tissue to a depth within the range of about 2 mm to about 5 mm.
In still another aspect of the invention, an RF ablation probe includes a handle, a shaft attached to the handle, and a distal section having a lumen therethrough, the distal section including a plurality of ports oriented on one side of the distal section, the distal section further including a plurality of RF electrodes, and a source of pressurized conductive fluid connected to the lumen of the probe.
In yet another aspect of the invention, a RF catheter includes a handle, a shaft attached to the handle, and a distal segment having a lumen therethrough, the distal segment including a plurality of ports oriented on one side of the distal section, the distal segment further including a plurality of RF electrodes, and a source of pressurized conductive fluid connected to the lumen of the catheter.
It is an object of the invention to provide a method for pre-treating tissue prior to RF ablation. It is a further object of the invention to provide a method that reduces the effective resistivity of tissues and thereby reduce power dissipation in those same tissues. Another object of the invention is to provide an improved RF ablation method that can form accurate and deep lesions within tissue. Another object of the invention is to proved a combination injection device/RF ablation device. The combined device can take the form of a probe or a catheter. These and other objects of the invention are described in detail below.